We examined the firing characteristics of single motor units in response to noninvasive magnetic brain stimulation in 17 patients with stroke in order to assess differences in motor unit responses between patients and normal human subjects. Recordings were made of the discharges of tonically active motor units in the abductor digiti minimi and biceps brachii muscles. The poststimulus time histograms and cumulative sums (cusums) of motor unit discharge were used to record changes in the firing probability of individual spinal motoneurones. For each period of changed firing probability its latency, duration and amplitude were determined and the presence of subpeaks was checked. A significantly higher stimulus intensity was usually required to excite motor units of stroke patients. In contrast to the rather stereotyped features of responses in normal subjects, the motor units of stroke patients exhibited great variability in response of different parameters. In some patients the period of increased firing probability was completely normal with short latency and duration, high amplitude and discernible subpeaks. In the others, it did not show discernible subpeaks, but otherwise appeared with normal latency. In most severely affected patients there was a delay in the onset of period of increased firing probability; its duration was increased and its amplitude decreased. In the same group of patients this period was typically preceded by the period of decreased firing
probability. The fact that higher stimulus intensity can activate larger motor cortex area and, possibly, also deeper brain structures suggests, that degenerated corticospinal fibres can be replaced through recruitment of unaffected descending pathways. Abnormalities of periods of increased firing probability can be explained by a number of mechanisms: axonal loss in the corticospinal tract, desynchronization of the descending impulses, recruitment of small diameter corticospinal fibres and perhaps also of some alternative, polysynaptic descending pathways. The period of decreased firing probability, which appeared with short latency, seems to represent the inhibition mediated by the Ia inhibitory interneurone. Noninvasive brain stimulation used in our study did not provide new reliable data on the mechanisms of stroke recovery.